The present invention relates to a coating composition comprising mineral pigment for coating of paper. The invention also relates to the use of particles in such a coating composition and a paper which exhibits a coating that comprises said coating composition.
Papers of different types are quite often coated in order to improve the quality of the paper. A conventionally used coating composition for this purpose is a dispersion comprising pigments, which pigments normally are inorganic, such as mineral pigments, but sometimes also organic. The coating composition may also comprise other additives.
Paper intended for printing, such as paper for illustrating books magazines, brochures and annual reports is quite often manufactured by coating a base paper in one or more coating units, followed by soft calendering. The final product matt or silk coated fine paper, will then exhibit a paper gloss of between 10 and 50% (measured for example with a Zehnter instrument angle 75 degrees). The used coating colours or coating compositions, are composed in a fairly conventional way, which means that the dominant pigments are calcium carbonate and clay. The mixing ratio varies from 0% to about 50%, preferably from 10% to about 50% of clay and even more preferred from 10% to about 40% of clay, the rest essentially being calcium carbonate. For a matt or silk coated paper at least 50% of the coating composition should consist of calcium carbonate, preferably at least 60% and even more preferred at least 70%. A ratio of 30% clay and 70% calcium carbonate is quite common. Also, talc may be used at a ratio of normally 5-15%.
Binders are normally used in a content of 8-15 parts per 100 parts inorganic pigments. Different types of latexes are normally used as binds but it is also common to use combinations of latex and water-soluble starch or just starch as a binder. The types of latexes could be all types of common latexes used for pigment coating on paper and board such as different copolymers based on styrene, butadiene, acrylates and methacrylates. As thickener and viscosity regulator water soluble polymers are used, for example carboxy methyl cellulose (CMC) of different molecular weights, derivatives of polyacrylates, polyvinyl alcohol, derivatives of polyacrylamid, starch etc. The amount is adjusted to reach a viscosity level suitable for coating with the referred coating techniques. Also, other additives, such as optical brighteners, dispersion agents, anti foaming agents, lubricants, etc may be used in the coating. The solids content of the coating composition is dependent on the type of coating composition and coating technique used, but is conventionally 55-70 weight percent.
Matt and silk coated fine papers suffer from a severe problem which occurs in the bookbindery or the like. This problem constitutes of transfer of ink from pictures and text to unprinted areas of the paper, which makes the paper look dirty. The problem occurs in various operations, such as folding and cutting, in the bookbindery or the like. The phenomenon has many names, but in the following the term xe2x80x9cink scuffxe2x80x9d will be used.
The degree of ink scuff can be determined in the laboratory after full scale printing, using a special equipment that scuffs the black full tone area of primed paper against unprinted paper of its own. The transfer of ink is measured as a xe2x80x9crub-offxe2x80x9d value, which for matt or silk coated fine papers usually lies between 6xc2x710xe2x88x922 and 7xc2x710xe2x88x922 after a drying time of 6 hours in the paper pile.
It is known that an increased amount of clay in the used coating composition and/or an addition of talc and lubricants (such as calcium stearate or waxes) in the coating composition will reduce ink scuff to some extent. The reduction achieved thereby is however in many cases not sufficient.
From EP 447 471 it is known to use starch grains in a coating composition for matt paper, in an amount of from 10 to 65% by weight based on its overall solids content. It has however been shown that matt paper coated accordingly exhibit unacceptably low print gloss.
From DE 26 05 575, it is known to use large amounts of starch in a latex coaxing which is especially developed for bank-notes, in order to fulfil the very special requirements for bank-notes.
From EP 825 296, it is known to use a coating composition comprising porous organic particles in an amount of 3-14% with a particle size of from 1.0 to 10 xcexcm. The object of the invention according to EP 825 296 is to provide a matte finished coated paper that has a surface coating of a pigment but is similar in appearance to an uncoated paper, which is superior in dot reproduction and is capable of developing colours clearly and setting inks for a short time and which has a coating that is resistant to scratch marks when scratched by something. Decreased ink scuff is however not an objective.
From U.S. Pat. No. 5,439,558 it is known to use a coating composition comprising starch granules in an amount of 2-25 weight % and with a particle size of 2-9 xcexcm. Moreover, the coating composition comprises 40-65% calcium carbonate and the gloss is greater than 40% as measured at an angle of 75xc2x0 according to Lehmann. The coated paper thus produced is not adapted for offset printing techniques and will thus not exhibit any ink scuff problems.
From WO 9920201 it is known to use a coating composition comprising at least 50% by volume hollow, organic polymeric particles with a particle size of about 1.0 to 5.0 xcexcm. The coating is intended to be used for papers with high paper and print gloss.